1. Field of the Invention
The present invention relates to a liquid crystal display device that effects display, making use of at least one of backlighting and ambient lighting.
2. Description of the Related Art
In recent years, liquid crystal display devices have been applied to a variety of fields including notebook PCs, monitors, car navigation systems, medium-sized and small-sized TVs, and mobile phones. In particular, a reflection-type liquid crystal display device requires no backlight, and has advantages such as low power consumption, thinness and lightness. To make use of these advantages, it is now under consideration to apply the reflection-type liquid crystal display device to a display for a portable device such as a mobile PC.
A reflection-type liquid crystal display device effects display by making use of ambient light. Like paper, the luminance of the display screen varies depending on the environment of illumination. In particular, nothing is visible in darkness. To solve this problem, special attention has recently been paid to a liquid crystal display device that uses a built-in light source as an auxiliary light source in a case where the ambient illumination environment is dark. Examples of this type of liquid crystal display include a backlit semi-transmission-type liquid crystal display device, wherein a built-in light source is disposed behind the display screen, and a frontlit reflection-type liquid crystal display device, wherein a built-in light source is disposed in front of the display screen.
In each of these liquid crystal display devices, in order to realize reduction in thickness and weight, while reducing power consumption, it is necessary to increase the efficiency of use of light as much as possible. Jpn. Pat. Appln. KOKAI Publication No. 2000-193962 (Patent No. 3015792), for instance, proposes a method which uses a polarizing reflection element such as a cholesteric liquid crystal as a reflection layer with high reflection efficiency. A cholesteric polarizing reflection element reflects circularly polarized light in a specific direction, which is included in the light incident on the reflection layer. Since the wavelength range of reflected light varies depending on the helical pitch of the cholesteric liquid crystal, light only in the desired wavelength range can be reflected by stacking a plurality of cholesteric liquid crystal layers with different pitches. In particular, by stacking six or more cholesteric liquid crystal layers, most of the light in the visible light range can be reflected. In this kind of cholesteric polarizing reflection device, compared to an ordinary metal reflection layer of, e.g. aluminum, the amount of absorbed light is small and the light use efficiency is high. The liquid crystal display device that uses the above-described polarizing reflection element can control the transmittance/reflectance of the polarizing reflection element. Thus, a semi-transmission-type liquid crystal display device with high light use efficiency can be provided.
On the other hand, reflection-type liquid crystal display devices or semi-transmission-type liquid crystal display devices, which can be formed of substantially the same material as conventionally used transmission-type liquid crystal display devices, have many merits such as sharing of equipment, reduction in cost of components and reduction in cost of development. These devices have already been put to practical use in mobile phones and personal digital assistants (PDAs). However, these reflection-type or semi-transmission-type liquid crystal display devices have lower light use efficiency than the liquid crystal display device that uses the above-described polarizing reflection device. Consequently, the reflection-type or semi-transmission-type liquid crystal display device has insufficient display characteristics, or consumes much energy in order to operate the light source with high luminance. In particular, in the semi-transmission-type liquid crystal display device, it is difficult to realize both transmission display characteristics and reflection display characteristics with sufficiently high light use efficiency.
Jpn. Pat. Appln. KOKAI Publication No. 11-242226, for instance, proposes various liquid crystal display modes as means for solving the above problem. For example, in a semi-transmission-type liquid crystal display device using a homogeneous mode, pixels are divided into a transmissive region and a reflective region which have different cell gaps, thereby to enhance the light use efficiency. In the homogeneous mode, since the liquid crystal molecules are aligned in the same direction, the structure is simple and optical compensation can easily be effected by using a retardation plate, etc. Therefore, display with high contrast and a wide viewing angle can be realized.
It is confirmed, however, that in the homogeneous mode, compared to a twisted nematic (TN) mode, etc., the variance in optical characteristics is large when there is a variance in the cell gap or retardation plate. It is necessary, therefore, to manage the cell gap, which corresponds to the transmissive/reflective regions, with a small margin in the manufacturing process, and to strictly manage the optical characteristics of the structural components. This poses serious problems in manufacture.